This research project will utilize organotypic cultures of innervated muscle to analyze mechanisms underlying the maturation and maintenance of normal neuromuscular relationships, with special emphasis on neurotrophic factors. We have demonstrated that fetal mouse spinal cord-dorsal root ganglion cross-sections cocultured with adult skeletal muscle form functional and structural interrelationships leading to the development of mature neuromuscular junctions as occur in situ. These organotypic cultures will now be utilized to study mechanisms underlying longterm maintenance of mature innervated muscle. Coordinated cytologic, histochemical, electrophysiologic and electron microscopic techniques will be utilized in these studies. The mature model will be compared with systematically simplified versions prepared by omission of specific cellular components, including appropriate competition experiments. Surgical denervation of mature cord-muscle cultures will be used as a basis for studies of trophic factors which may alter denervated muscle atrophy. This basic rodent cord-muscle culture system will also be applied to problems in human muscle development by using explants of normal and pathologic human muscle obtained from biopsies. The greatly enhanced maturation of human muscle achieved through co-culture with mouse cord may reveal developmental deficits associated with progressive dystrophic disorders.